In the industrial preparation of isocyanates, distillation residues are produced, which require further working up. These distillation residues comprise, inter alia, polymeric products and still contents of monomeric isocyanate. The present invention relates to a process which renders it possible to recover this content of monomeric isocyanate in the distillation residue in a high yield in a simple manner.
The state of the art for treatment of the said distillation residues of isocyanate preparation describes various processes. General aims of treatment of residues are maximizing of the isocyanate yield, minimizing of the amount of residue produced and an as appropriate as possible inexpensive and simple utilization of the amount of residue which is no longer usable for the isocyanate preparation process.
The following processes are chiefly known:
In principle, the distillation residue can be burned continuously or discontinuously. The process is technically simple and can be employed for generating service steam if a plant for thermal utilization which is suitable for this exists close to the isocyanate production plant, in order to ensure disposal via a pipeline connection. The great disadvantage of this process, however, is the loss in yield caused by the fact that the distillation residue always also comprises contents of the valuable product, which is burned at the same time. If the distillation of the isocyanate were to be operated such that the isocyanate were removed completely or almost completely from the bottom product, a solid residue which can be processed only with great difficulty would remain. To avoid this, the distillation conditions are conventionally chosen such that the bottom product of the distillation column remains liquid, which is only successful, however, if this still comprises a substantial content of the desired isocyanate, which is therefore inevitably co-fed to the combustion.
To minimize the losses in isocyanate yield, the distillation residue can be transferred into a stirred and heated tank and mixed with high-boiling hydrocarbons, preferably bitumen, which are inert under the distillation conditions, in order to distil off as completely as possible the free isocyanate still present in the residue (EP 0 548 685 A2). The residue which remains, which has been freed from isocyanate, can be discharged as a free-flowing solid and fed to a combustion. Disadvantages of this process are, in addition to the use of a substance foreign to the process (bitumen), losses in yield due to polymerization of the isocyanate, since the process includes long dwell times at a high temperature.
A further process for separating off the isocyanate residue is characterized by the use of kneader driers (EP 0 626 368 A1). In this process, the heated and stirred tanks described above are replaced by kneader driers. By employing, for example, bitumen, as in the abovementioned example the residue which remains is obtained as a free-flowing solid which can be employed as a fuel, for example in cement works. The advantage of this process over that mentioned above is an increase in yield, but the higher investment costs due to the more involved technique may be seen as a disadvantage. The use of mechanically moved parts moreover necessarily leads to a higher outlay on maintenance.
EP 0 699 659 A2 describes a process and a device for separating off a solid residue from a solution of the residue in vaporizable valuable substances and/or solvents with the addition of up to 20 wt. % of high-boiling hydrocarbons which are inert under the vaporization conditions of the valuable substances, heating of the mixture to the vaporization temperature in vacuo, whereupon the valuable substances vaporize and are stripped off and condensed and the residue is obtained as a free-flowing solid, the residue solution being introduced on to a stirred bed of granular, solid material kept at the vaporization temperature. A disadvantage here is the additional use of high-boiling solvents, which have to be worked up in a further process.
The patent literature also describes processes in which isocyanate distillation residues are reacted chemically in order to obtain industrially usable valuable substances, such as, for example, the reaction of residue from the preparation of toluylene-diisocyanate with alkanolamine (U.S. Pat. No. 5,902,459) or with isocyanates of the diphenylmethane series (DE 42 11 774 A1, U.S. Pat. No. 3,694,323).
The hydrolysis of isocyanate distillation residues with water for the purpose of recovery of the starting amine, in particular in the preparation of toluylene-diisocyanate (TDI in the following) is a field which has already been worked on for a relatively long time and is described, for example, in U.S. Pat. No. 3,128,310, U.S. Pat. No. 3,331,876, GB 795,639, DE 27 03 313 A1 and EP 1 935 877 A1. In the processes cited, isocyanate distillation residue is hydrolysed with water under increased pressure and at elevated temperature. In this context, some of the residue is converted into the original amine, which can be recycled into the phosgenation process again after appropriate working up and therefore leads to minimizing of the residue. The unsatisfactory part of this process is that some of the valuable product, isocyanate, is hydrolysed to the starting substance again, and must be phosgenated again. As a result, the isocyanate contained in the residue is indeed fed to an appropriate substance utilization, but it would be desirable to be able to recover the isocyanate as such from the residue.
EP 1 413 571 A1 and EP 1 371 633 A1 are concerned with optimizing the working up of TDI by employing a dividing wall column in the distillation, which results inter alia in a reduction of the content of TDI in the bottom product. Here also, however, production of an isocyanate-containing distillation residue cannot be prevented.
EP 0 017 972 A1 describes a process for separating off TDI and/or higher-boiling solvents from distillation residues which are formed in the preparation of TDI by phosgenation of toluylenediamine, by evaporation in a fluidized bed at temperatures of from 140 to 280° C. For this purpose, the distillation residue is heated up and is sprayed at temperatures of from 50 to 300° C. via an introduction device, for example a two-component nozzle or several one-component nozzles, into a fluidized bed of small initially introduced particles of particular particle sizes (0.5 to 5,000 μm) and a fluidizing gas. The initially introduced particles are residue which has already been worked up and is substantially free from the valuable substance (cf. for example p. 6,1. 8 to 21 and p. 7,1. 15 to 17 and 32 to 36). In this process, the drops of the distillation residue which are introduced into the fluidized container by means of the introduction device are sprayed on to the surface of the initially introduced particles and spread there, which leads to vaporization of the valuable product (TDI and/or higher-boiling solvent) and leads to the build-up of shell-like granules of residue which is free from the valuable substance. The vaporization of the valuable product takes place on the initially introduced particles. The residue to be worked up is already depleted in isocyanate before introduction into the fluidized reactor to the extent that it must be injected into the fluidized bed at least partly as a melt (p. 4,1. 20 to 23), since too high a viscosity impedes the atomization process. A disadvantage is that the particle size of the initially introduced particles must be established in an involved manner (cf. for example the guiding of the particles in the figure via the discharge (5), the “grading device” (18) and the comminuting device (8) back into the fluidized reactor).
Such a granulation process as a rule does not have relatively long cycle times and must be shut down after certain intervals of time for the purpose of intermediate cleaning. For the present case of working up of isocyanate-containing residues this is a disadvantage due to the required inertization of the reaction space and the high temperatures as well as the start-up problems. A stationary process requires that a bed having a specific particle size distribution is established again in the fluidized bed. For example, if a fine content is lacking, there is the danger that the reactor runs empty after the original bed content has grown, while if the fine content is too high, the formation of too large a bed can take place and the process can collapse as a result of the high mechanical wear which then occurs. Such granulation processes are therefore usually closely observed by regular sampling and/or visual observation possibilities. Both measures not only require an increased outlay on manual supervision, but also are more difficult to realize due to the process conditions required, in particular due to the inertization and high temperatures already mentioned above.
As a result of the abovementioned disadvantages of the state of the art, there was a demand for a simple and economical process for the production of monomeric isocyanate from a distillation residue produced in isocyanate preparation. This should be distinguished in particular by operating stability with a minimized outlay on supervision, in addition to the highest possible yield of monomeric isocyanate.